This application relates to a refrigerant system wherein a pulse width modulation controlled suction valve is provided, along with an additional refrigerant flow path and a suction modulation valve. The suction modulation valve can be opened to a small degree such that when the pulse width modulation valve is closed, some refrigerant still can leak through to the suction modulation valve and avoid a potentially harmful situation in the compressor due to very low suction pressure.
Refrigerant systems are known, and are utilized to condition a secondary fluid. As an example, an air conditioning system cools and typically dehumidifies air being delivered into a climate controlled environment.
Refrigerant systems generally include a compressor compressing refrigerant and delivering that refrigerant through a discharge line to a downstream heat exchanger. From this downstream heat exchanger, refrigerant passes through an expansion device and then to another heat exchanger. The refrigerant is then returned to the compressor.
At times, it is desirable to reduce the capacity provided by the refrigerant system to match the required capacity of the conditioned environment.
One known method for reducing capacity is the use of a pulse width modulation controlled suction valve. A pulse width modulation controlled suction valve is a valve that is rapidly cycled between open and closed positions. When the pulse width modulation control valve is closed, it blocks flow of refrigerant to the compressor, and thus essentially eliminates the compression of refrigerant. Such a blockage of refrigerant flow reduces the provided capacity and power consumption.
One challenge with regard to utilizing pulse width modulation control for a suction valve and reducing the pressure to the lowest possible value, for the purpose of reducing delivered capacity, is that, when the suction valve is in the closed position, to reduce capacity to its minimum, the pressure within a compressor shell may decrease to an extremely low value to be effectively near complete vacuum. It is undesirable to have a pressure below a minimum predetermined pressure threshold in the compressor shell, as this may sometimes lead to an undesirable condition so-called “corona discharge.” Also, the compressor motor can overheat causing the discharge temperature to be above the specified safe reliability limit. A “corona discharge” phenomenon can occur over the electrical terminals, and may lead to a deterioration of the terminals, and consequently to compressor failure. This minimum pressure is typically around 0.5 psia.
In the past, this problem has been addressed by having a “leaky” pulse width modulating valve (for example, a valve with a small opening in its stem), or a bypass loop has been installed around the pulse width modulating valve. The small opening through the “leaky” pulse width modulating valve or the small opening in the bypass line is sized to maintain the pressure in the compressor suction compartment above a certain minimum value mentioned above. However, due to changing operating conditions, the size of this opening would also need to be changed in relation to the operating conditions to achieve optimum performance. Since it is impossible to change the size of the opening for a fixed size restriction, it leads to less than optimum performance. The pressure needs to be above the minimum threshold value for all operating conditions, and this led to a situation where, for some operating conditions, the compressor was operating at suction pressures that were well above the minimum acceptable limit. This in turn led to lower system efficiency, because the pressure was too high to assure the efficient operation at such “off-design” conditions. Thus, a need exists to adjust the suction pressure to be just above the minimum acceptable level for all operating conditions. In other words, in the prior art, the pressure could not be controlled to a minimum acceptable level for all operating conditions, while using the fixed size restriction, causing the pressure to be higher than desired for some of the operating conditions, and leading to lower efficiency in these instances. Thus, it would be desirable to maintain the pressure at the lowest possible level when a suction valve is pulse width modulated to a closed position for all the operating conditions. Also the failure of the pulse width modulated valve, in the prior art systems, would often lead to unacceptable performance. If, for example, the pulse width modulated valve is to fail in the closed position, the compressor damage would be likely due to overheating and the system would deliver no capacity or nearly no capacity. If the pulse width modulated valve, on the other hand, is to fail in the open or any intermediate position, then the system performance would also often be unacceptable, as the capacity can no longer be modulated.